In making anhydrous dicalcium phosphate (hereafter DCPA) by prior art processes, several alternatives have been followed. In one process (Process I) and aqueous lime slurry is added to an aqueous phosphoric acid solution to form hydrated dicalcium phosphate or dicalcium phosphate dihydrate (hereafter DCPD) which is separated from the aqueous liquid phase and then dehydrated by heating. In another process, (Process II) a soluble salt of phosphoric acid, such as diammonium phosphate, and a soluble calcium salt, such as calcium chloride, are mixed together in aqueous solution to from hydrated dicalcium phosphate under acidic conditions. The resulting aqueous medium is then heated near 100.degree. C. for a period of time sufficient to convert the dicalcium phosphate to the anhydrous form. This product is then separated from the aqueous medium and dried.
A process similar to Process II is used to produce anhydrous calcium hydrogen phosphate as in U.S. Pat. No. 3,635,860 to Joseph A. G. Bruce et al, issued Jan. 18, 1972. In accordance with this patent the anhydrous product can be converted to a calcium pyrophosphate by heating to about 500.degree.-600.degree. C. and this product can be used to produce luminescent phosphors.
The DCPA produced in accordance with Process I can be converted to a calcium pyrophosphate (CPP) which contains at least 70% of .beta. phase material by calcination at a temperature of about 700.degree. C. However, when such calcination is carried out in commercial scale equipment, the CPP obtained will vary in properties from batch to batch and may not always be suitable as a dentifrice abrasive material. Thus such a material, in order to be commercially acceptable, should have good cleaning power, relatively low radioactive dentine enamel abrasion (RDA), high sodium fluoride compatibility, and stannous (tin) compatibility, resulting in a relatively low RDA to stannous compatibility ratio. If the products produced do not have such properties consistently the cost of acceptable product is obviously going to be greater than it would be if acceptable product could be produced consistently. The reason for variance in CPP properties from batch to batch are not clearly understood, but are believed to be influenced by crystal qualities (including crystal size), conditions of dehydration and conditions existing during the conversion of DCPA to CPP.
The applicants have found that the vital dentifrice abrasive properties of CPP can be controlled more consistently than has been possible heretofore in commercial practice if hydrated dicalcium phosphate is first formed by adding an aqueous phosphoric acid solution to an aqueous lime CaO or Ca(OH).sub.2 slurry with adequate agitation and holding the resultant product in an aqueous medium under elevated temperature conditions sufficient to dehydrate the dicalcium phosphate in such medium and form DCPA. This product can be used per se as a dentifrice polishing agent or it can be converted by calcination to the .gamma. or .beta. form of CPP depending on the calcination temperatures used. The .beta. form of CPP produced by the present process is substantially consistently useful as a polishing agent (or abrasive) in dentifrice compositions.
It can be seen from the foregoing that the present process differs from Process I, supra, primarily in the manner of mixing phosphoric acid and lime slurry to produce hydrated DCP and DCPA. The consequence of this difference will be apparent from the subsequent disclosure. One of the essential differences between the present processes and Process II is the starting materials employed to prepare the calcium phosphate products. The materials used in Process II may be suitable for making phosphors, but are not economical for making dentifrice polishing agents.